Gas electrodes, in which a gas is passed in contact with a suitable electrode conductor in the presence of an electrolyte solution are well known. Amongst several practical employments, such assemblies even find occasional application for use as reference electrodes.
In their typical and most popular utilizations, gas electrodes function in systems capable of generating electricity (such as fuel cells and the like) or for electrolysis purposes in which the electrode performs as a depolarized cathode (as in chlor-alkali and the like or equivalent and analogous product manufacturing operations). These gas electrode installations implement electrochemical reactions involving the interaction with and between three individual phases of a gas, a liquid (usually aqueous) electrolyte and electrons provided directly from a solid conductor surface, all of which are in necessary simultaneous respectively mutual contact in order to accomplish desired results. So that, with and for given unit geometric volumes, maximization can be realized of the available surface area on which the requisite three-phase contact is believed according to at least one theory to occur (thereby possibilitating greater current density obtentions with the given units), modern gas electrodes are made to be porous. Because, according to the indicated theoretical presumption, the reaction takes place on the interior interstitial surfaces of the porous electrode bodies, it is consequently felt to be important that the three-phase contact area for the reaction be kept in a stable and at least relatively precise location.
The means so far developed for localizing the site of the three-phase reaction within the passageways of porous electrode bodies have included one of three applicable ways of so doing, namely:
(A) To treat the pore interiors on the gas side of the electrode with a material (such as "Teflon", a fluorinated ethylene polymer) which is not wet by the electrolyte so that liquid is prevented from penetrating entirely through the electrode.
(B) To maintain the desired regional three-phase contact by very careful balance between gas pressure exerted and capillary pressure generated with the electrolyte solution which is possible by use of a usually metallic, porous electrode body fabricated so as to have a very narrow distribution of pore sizes.
(C) To use a dual porosity structure for the electrode body of, again, usually metallic construction wherein the layer designed to face the electrolyte has smaller pores than those in the adjacent, complementary layer by means of which it is possible to apply a gas pressure through the larger pored layer that is greater than the median electrolyte capillary pressure in the large pores but smaller than that in the small pore layer so as to maintain the three-phase contact sector within the interstitial passageways at least approximately in the vicinity of the joinder boundary of the layers; this sort of construction being easier to make than the variety described in the above Paragraph (B) since not such a difficult to fabricate, narrow pore size distribution is demanded for each of the layers in the electrode.
Of historic note and interest, dual porosity electrodes of the type particularized in Paragraph (C) above were employed in various apparatus used in the so-called "Apollo" Space Program conducted by the National Aeronautics and Space Administration of the United States of America.
Various aspects relevant to the use of gas electrodes in galvanic and electrolysis mode applications, including oxygen depolarized cathodes in electrolytic cells, are amply demonstrated in, inter alia, U.S. Pat. Nos. 1,474,594; 2,273,795; 2,680,884; 3,035,998; 3,117,034; 3,117,066; 3,262,868; 3,276,911; 3,316,167; 3,377,265; 3,507,701; 3,544,378; 3,645,796; 3,660,255; 3,711,388; 3,711,396; 3,767,542; 3,864,236; 3,923,628; 3,926,769; 3,935,027; 3,959,112; 3,965,592; 4,035,254; and 4,035,255; and Canadian Pat. No. 700,933. A good description of dual porosity electrodes for fuel cell usage is set forth at pages 53-55 of "Fuel Cells" by G. J. Young (Reinhold Publishing Company, N.Y., 1960). All of the noted citations and all of the contents thereof are herein incorporated by reference, taking into account that the complete body of literature available as to this general subject matter (including dual porosity electrodes) is already vast and multitudinous.
As indicated, oxygen gas-bearing depolarized electrodes are of especial interest in commercial, large-scale chlor-alkali operations and analogous electrolyzations of other alkali metal or acid halides. In the electrolysis of common salt brine, for example, the reaction at the depolarized cathodic oxygen electrode in the alkaline media of the catholyte is: EQU O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4O H.sup.- with E.degree.=0.401 v.
In comparison, the cathode reaction in a traditionally conventional chlor-alkali cell is: EQU 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2O H.sup.- with E.degree.=-0.828 v.
Thus, the use of an oxygen gas-bearing depolarized, dual porosity electrode for chlor-alkali electrolytic cells brings about a theoretically achievable electrical potential requirement saving of 1.229 volts. This, for practical purposes, translates to the possibility of very substantial reduction in and economization of power costs when reckoned from the basis of the usual relatively low voltages (frequently in the range of ca. 2-5 volts) at which a typical chlor-alkali diaphragm cell operates in order to effect the desired electrolysis and accommodate associated overvoltage requirements.
Nonetheless, there are certain considerable difficulties involved in the utilization for large-scale, commercial manufacturing purposes (as in the chlor-alkali trade) of dual porosity gas electrodes. Probably most significantly problematical and perplexing of these is the frequent occurrence of bubbling or leaking of reactant gas under full electrolyte restraining pressure through at least the upwardly disposed electrolyte facing portions of a vertically emplaced electrode in truly big cell assemblies. In many commercial installations, the electrolyte is often contained for reasons of practical necessity in considerable depth (frequently as deep as 4 feet--ca. 1.2 meters--and deeper). With a head of such magnitude, the catholyte exerts a substantial hydraulic pressure (usually at least 1 psig and often on the order of 2-3 psig--ca. 0.69 to 1.38-2.07 dynes/cm.sup.2 --and greater). In other words, tall and massive electrodes introduce a new and important factor with which to contend; this being the not-inconsequential liquid pressure effect on the electrode simply due to the height of the electrolyte in the cell and its corresponding magnified head pressure. If the gas pressure is reduced to avoid bubbling in the upper portions of the electrode, the increasingly pressurized liquid towards the lower electrode portions overcomes its restraint by the applied gas. It then invariably leaks through the pores in that area causing other major and contrary problems of electrolyte loss into the bottom of the gas chamber from which the gas is pressed (or even into the gas supply system). This often tends to inoperate or at least considerably diminish the effectiveness and productive capacity of the cell.
Leaking is, plainly, extremely undesirable. Not only does it tend to materially interfere with and diminish overall reaction efficacy (since it loses the advantageous electrochemical and reduced voltage requirement reasons for keeping the reaction in a desirable stable interstitial area), it occasions, amongst other things, escape of reaction gas which is either lost or, if collected, must be handled through recovery and reprocessing units for subsequent re-use. In any event, leaking unavoidably and to quite appreciable extents can increase the cost of the operation.
Analogous considerations apply as to leakage and other problems in large-scale fuel cells and the like battery devices involving use of dual porosity electrodes.
The heretofore known and employed dual porosity electrodes have been and are generally of relatively small size of invariably less than about 18 inches (ca.45.7 centimeters) in height when vertically utilized, and usually even much shorter than that. In cells of such relatively small-scale dimensional magnitudes, the hydraulic electrolyte pressure heads involved are negligible and of no practical concern or consequence insofar as is relevant to gas leakage and associated problems. Rarely do they substantially approach even as small as a 1 psig value. In fact, the diminutive dual porosity gas electrodes which have so far appeared have not been uncontrollably hampered by bubbling or leakage problems because of the explained, embodied dimensional limitations which avoid serious inherent susceptibility thereto. Thus, there really has been no specialized prior art address to the encumbrance and impediments of bubbling and/or leaking in dual porosity electrodes of massive and relatively large-scale body design and construction.
The basic characteristics and operational principles and limitations of porous, including dual porous, electrode design and utilization practice are so widely comprehended by those skilled in the art that further elucidation thereof and elaboration thereon is unnecessary for thorough understanding and recognition of the advance contributed and made possible to achieve by and with the development(s) of the present invention.